Back to EveryPatent.com
United States Patent |
6,157,278
|
Katznelson
,   et al.
|
December 5, 2000
|
Hybrid magnetic apparatus for use in medical applications
Abstract
Permanent magnet assemblies and hybrid magnetic apparatus are disclosed for
use in medical applications, particularly permanent magnet assemblies for
use in Magnetic Resonance Imaging (MRI) and/or Magnetic Resonance Therapy
(MRT) to produce a volume of substantially uniform magnetic field in a
restricted part of the patient's body in a region either adjacent to the
surface of one permanent magnet assembly or between a set of a first and
second permanent magnet assemblies, leaving open access to the patient's
body. The assemblies consist of a plurality of annular concentric magnets
spaced-apart along their axis of symmetry. A method for constructing such
annular permanent magnetic assemblies is disclosed, using equi-angular
segments permanently magnetized. The hybrid magnetic apparatus includes an
electromagnet flux generator for generating a first magnetic field in the
volume, and permanent magnet assemblies for generating a second magnetic
field superimposed on the first magnetic field for providing a
substantially homogenous magnetic field having improved magnitude within
the volume.
Inventors:
|
Katznelson; Ehud (Ramat Yishai, IL);
Zuk; Yuval (Haifa, IL);
Rotem; Haim (Kfar Klil, IL)
|
Assignee:
|
Odin Technologies Ltd. (Yokneam, IL)
|
Appl. No.:
|
274671 |
Filed:
|
March 24, 1999 |
Current U.S. Class: |
335/296; 324/319; 324/320; 335/298; 335/299; 335/306 |
Intern'l Class: |
H01F 003/00; H01F 007/22; G01V 003/00; G01R 033/20 |
Field of Search: |
335/216,296-306
324/318-320
600/410,407,421,422
|
References Cited
U.S. Patent Documents
4829252 | May., 1989 | Kaufman | 324/309.
|
4875485 | Oct., 1989 | Matsutani.
| |
5134374 | Jul., 1992 | Breneman et al. | 324/319.
|
5304933 | Apr., 1994 | Vavrek et al. | 324/318.
|
5309106 | May., 1994 | Miyajima et al. | 324/318.
|
5332971 | Jul., 1994 | Aubert | 324/319.
|
5410287 | Apr., 1995 | Laskaris et al. | 335/216.
|
5428292 | Jun., 1995 | Dorri et al. | 324/319.
|
5517169 | May., 1996 | Laskaris et al. | 335/301.
|
5565831 | Oct., 1996 | Dorri et al. | 335/216.
|
5568102 | Oct., 1996 | Dorri et al. | 335/216.
|
5574417 | Nov., 1996 | Dorri et al. | 335/216.
|
5880661 | Mar., 1999 | Davidson et al. | 335/306.
|
5900793 | May., 1999 | Katznelson et al. | 335/296.
|
Primary Examiner: Donovan; Lincoln
Assistant Examiner: Barrera; Raymond
Attorney, Agent or Firm: Eitan, Pearl, Latzer & Cohen-Zedek
Parent Case Text
RELATED U.S. APPLICATIONS
This application is a continuation in part of U.S. patent application Ser.
No. 08/898,773 to Katznelson et al., titled "PERMANENT MAGNETIC ASSEMBLIES
FOR USE IN MEDICAL APPLICATIONS", filed Jul. 23, 1997, now U.S. Pat. No.
5,900,793, assigned to the assignee of the present invention and
incorporated herein by reference in its entirety.
Claims
We claim:
1. Open magnetic apparatus for use in an MRI or IMRI device to produce a
predetermined volume of substantially uniform magnetic field directed
parallel to an axis of symmetry of said volume, the apparatus comprising:
a first electromagnet assembly disposed at a first position along said
axis, said first electromagnet assembly includes at least a first
electromagnet coil, said at least first coil is radially symmetric with
respect to said axis;
a second electromagnet assembly disposed at a second position spaced apart
from said first position of said electromagnet assembly along said axis,
said second electromagnet assembly includes at least a second
electromagnet coil, said at least second coil is radially symmetric with
respect to said axis, said at least first coil and said at least second
coil are equidistant from the center of said volume, said first
electromagnet assembly and said second electromagnetic assembly are
adapted for generating a first magnetic field within said volume;
a first permanent magnet assembly positioned at a third position disposed
between said first position and said second position along said axis, said
first permanent magnet assembly has an inner surface facing said volume
and an outer surface facing said first electromagnet assembly, said first
permanent magnet assembly includes,
a first permanent magnet having an outer diameter, said first permanent
magnet is radially symmetric with respect to said axis and has a first
magnetization direction parallel to said axis,
at least a second annular permanent magnet coaxial with said first
permanent magnet, said at least second permanent magnet has an inner
diameter larger than said outer diameter of said first permanent magnet,
said second permanent magnet is radially symmetric with respect to said
axis and has a second magnetization direction parallel to said axis, and
a first low magnetic permeability frame for supporting said first permanent
magnet and said at least second permanent magnet; and
a second permanent magnet assembly opposed to said first permanent magnet
assembly, said second permanent magnet assembly is positioned at a fourth
position spaced apart from said third position along said axis, said
second permanent magnet assembly has an inner surface facing said volume
and an outer surface facing said second electromagnet assembly, said
second permanent magnet assembly includes,
a third permanent magnet having an outer diameters, said third permanent
magnet is radially symmetric with respect to said axis and has a
magnetization direction identical to said first magnetization direction,
at least a fourth permanent magnet coaxial with said third permanent
magnet, said at least fourth permanent magnet has an inner diameter larger
than said outer diameter of said third permanent magnet, said fourth
permanent magnet is radially symmetric with respect to said axis and has a
magnetization direction identical to said second magnetization direction,
and
a second low magnetic permeability frame for supporting said third
permanent magnet and said at least fourth permanent magnet,
wherein the inner surface of said first permanent magnet assembly and the
inner surface of said second permanent magnet assembly define an open
region therebetween, said volume is disposed within said open region, said
third position of said first permanent magnetic assembly and said fourth
position of second permanent magnetic assembly are equidistant along said
axis from the center of said volume and wherein said first permanent
magnetic assembly and said second permanent magnetic assembly are adapted
to generate a second magnetic field superimposed on said first magnetic
field to provide said substantially uniform magnetic field within said
volume.
2. The magnetic apparatus according to claim 1, wherein said first
magnetization direction is oriented parallel to said second magnetization
direction.
3. The magnetic apparatus according to claim 1, wherein said first
magnetization direction is oriented anti-parallel to said second
magnetization direction.
4. The magnetic apparatus according to claim 1, wherein said at least first
electromagnet coil and said at least second electromagnet coil are
operatively arranged as a Helmholtz pair.
5. The magnetic apparatus according to claim 1, wherein said first low
magnetic permeability frame is adapted for varying the position of at
least said first permanent magnet along said axis, and said second low
magnetic permeability frame is adapted for varying the position of at
least said third permanent magnet along said axis.
6. The magnetic apparatus according to claim 1, wherein said first low
magnetic permeability frame is adapted for varying the tilt angle of at
least said first permanent magnet with respect to said axis, and said
second low magnetic permeability frame is adapted for varying the tilt
angle of at least said third permanent magnet with respect said axis.
7. The magnetic apparatus according to claim 1, wherein said at least first
electromagnet coil and said at least second electromagnet coil are
resistive electromagnet coils.
8. The magnetic apparatus according to claim 1, wherein said at least first
electromagnet coil and said at least second electromagnet coil are
superconducting electromagnet coils.
9. The magnetic apparatus according to claim 1, further including a first
gradient coil assembly disposed between said outer surface of said first
permanent magnet assembly and said first electromagnet assembly, and a
second gradient coil assembly disposed between said outer surface of said
second permanent magnet assembly and said second electromagnet assembly.
10. The magnetic apparatus according to claim 9, wherein each of said first
gradient coil assembly and said second gradient coil assembly includes one
or more gradient coils selected from an x-gradient coil, a y-gradient
coil, a z-gradient coil and any combination thereof.
11. The magnetic apparatus according to claim 10, wherein each of said
first gradient coil assembly and said second gradient coil assembly is a
multi-layer printed circuit assembly, and wherein at least one of said
x-gradient coil, y-gradient coil and z-gradient coil of each of said first
gradient coil assembly and said second gradient coil assembly is a
substantially planar printed circuit coil.
12. The magnetic apparatus according to claim 1, wherein said first and
said third permanent magnets are selected from a disc-like permanent
magnet having a circular cross section in a plane perpendicular to said
axis, a regular right polygonal prism-like permanent magnet having a
regular polygonal cross-section in a plane perpendicular to said axis and
having N sides, a ring-like annular permanent magnet and a annular right
regular polygonal permanent magnet having N sides.
13. The magnetic apparatus according to claim 12, wherein N is equal to or
larger than eight.
14. The magnetic apparatus according to claim 1, wherein said at least
second and said at least fourth permanent magnets are selected from a
ring-like annular permanent magnet and an annular right regular polygonal
permanent magnet having N sides.
15. The magnetic apparatus according to claim 14, wherein N is equal to or
larger than eight.
16. The magnetic apparatus according to claim 1, wherein said at least
first electromagnet coil and said at least second electromagnet coil are
selected from a circular coil and a regular polygonal shaped coil having N
sides.
17. The magnetic apparatus according to claim 16, wherein N is equal to or
larger than eight.
18. The magnetic apparatus according to claim 1, wherein said first
permanent magnet and said third permanent magnet comprise a plurality of
permanently magnetized segments attached to adjacent segments using a
non-conductive adhesive, so as to reduce eddy currents.
19. The magnetic apparatus according to claim 18, wherein said segments are
equi-angular segments.
20. The magnetic apparatus according to claim 1, wherein said at least
second annular permanent magnet and said at least fourth annular permanent
magnet comprise a plurality of permanently magnetized segments attached to
adjacent segments using a non-conductive adhesive, so as to reduce eddy
currents.
21. The magnetic apparatus according to claim 20, wherein said segments are
equi-angular segments.
22. A method for constructing an open magnetic apparatus for use in an MRI
or IMRI device to produce a predetermined volume of substantially uniform
magnetic field directed parallel to an axis of symmetry of said volume,
the method comprising the steps of:
providing a first electromagnet assembly disposed at a first position along
said axis, said first electromagnet assembly includes at least a first
electromagnet coil, said at least first coil is radially symmetric with
respect to said axis;
providing a second electromagnet assembly disposed at a second position
spaced apart from said first position of said electromagnet assembly along
said axis, said second electromagnet assembly includes at least a second
electromagnet coil, said at least second coil is radially symmetric with
respect to said axis, said at least first coil and said at least second
coil are equidistant from the center of said volume, said first
electromagnet assembly and said second electromagnetic assembly are
adapted for generating a first magnetic field within said volume;
providing a first permanent magnet assembly positioned at a third position
disposed between said first position and said second position along said
axis, said first permanent magnet assembly has an inner surface facing
said volume and an outer surface facing said first electromagnet assembly,
said first permanent magnet assembly includes,
a first permanent magnet having an outer diameter, said first permanent
magnet is radially symmetric with respect to said axis and has a first
magnetization direction parallel to said axis,
at least a second annular permanent magnet coaxial with said first
permanent magnet, said at least second permanent magnet has an inner
diameter larger than said outer diameter of said first permanent magnet,
said second permanent magnet is radially symmetric with respect to said
axis and has a second magnetization direction parallel to said axis, and
a first low magnetic permeability frame for supporting said first permanent
magnet and said at least second permanent magnet; and
providing a second permanent magnet assembly opposed to said first
permanent magnet assembly, said second permanent magnet assembly is
positioned at a fourth position spaced apart from said third position
along said axis, said second permanent magnet assembly has an inner
surface facing said volume and an outer surface facing said second
electromagnet assembly, said second permanent magnet assembly includes,
a third permanent magnet having an outer diameter, said third permanent
magnet is radially symmetric with respect to said axis and has a
magnetization direction identical to said first magnetization direction,
at least a fourth permanent magnet coaxial with said third permanent
magnet, said at least fourth permanent magnet has an inner diameter larger
than said outer diameter of said third permanent magnet, said fourth
permanent magnet is radially symmetric with respect to said axis and has a
magnetization direction identical to said second magnetization direction,
and
a second low magnetic permeability frame for supporting said third
permanent magnet and said at least fourth permanent magnet,
wherein the inner surface of said first permanent magnet assembly and the
inner surface of said second permanent magnet assembly define an open
region therebetween, said volume is disposed within said open region, said
third position of said first permanent magnetic assembly and said fourth
position of second permanent magnetic assembly are equidistant along said
axis from the center of said volume and wherein said first permanent
magnetic assembly and said second permanent magnetic assembly are adapted
to generate a second magnetic field superimposed on said first magnetic
field to provide said substantially uniform magnetic field within said
volume.
23. A method for providing a substantially homogenous magnetic field within
a volume disposed in an open region within a magnetic apparatus the method
comprising the steps of:
providing a first electromagnet assembly disposed at a first position along
said axis, said first electromagnet assembly includes at least a first
electromagnet coil, said at least first coil is radially symmetric with
respect to said axis, and a second electromagnet assembly disposed at a
second position spaced apart from said first position of said
electromagnet assembly along said axis, said second electromagnet assembly
includes at least a second electromagnet coil, said at least second coil
is radially symmetric with respect to said axis, said at least first coil
and said at least second coil are equidistant from the center of said
volume;
providing a first permanent magnet assembly positioned at a third position
disposed between said first position and said second position along said
axis, said first permanent magnet assembly has an inner surface facing
said volume and an outer surface facing said first electromagnet assembly,
said first permanent magnet assembly includes,
a first permanent magnet having an outer diameter, said first permanent
magnet is radially symmetric with respect to said axis and has a first
magnetization direction parallel to said axis,
at least a second annular permanent magnet coaxial with said first
permanent magnet, said at least second permanent magnet has an inner
diameter larger than said outer diameter of said first permanent magnet,
said second permanent magnet is radially symmetric with respect to said
axis and has a second magnetization direction parallel to said axis, and
a first low magnetic permeability frame for supporting said first permanent
magnet and said at least second permanent magnet, and
a second permanent magnet assembly opposed to said first permanent magnet
assembly, said second permanent magnet assembly is positioned at a fourth
position spaced apart from said third position along said axis, said
second permanent magnet assembly has an inner surface facing said volume
and an outer surface facing said second electromagnet assembly, said
second permanent magnet assembly includes,
a third permanent magnet having an outer diameter, said third permanent
magnet is radially symmetric with respect to said axis and has a
magnetization direction identical to said first magnetization direction,
at least a fourth permanent magnet coaxial with said third permanent
magnet, said at least fourth permanent magnet has an inner diameter larger
than said outer diameter of said third permanent magnet, said fourth
permanent magnet is radially symmetric with respect to said axis and has a
magnetization direction identical to said second magnetization direction,
and
a second low magnetic permeability frame for supporting said third
permanent magnet and said at least fourth permanent magnet,
wherein the inner surface of said first permanent magnet assembly and the
inner surface of said second permanent magnet assembly define an open
region therebetween, said volume is disposed within said open region, said
third position of said first permanent magnet assembly and said fourth
position of second permanent magnetic assembly are equidistant along said
axis from the center of said volume, and wherein said first permanent
magnetic assembly and said second permanent magnetic assembly generate a
first permanent magnetic field in said volume;
electrically energizing said first electromagnet assembly and said second
electromagnetic assembly to provide a second magnetic field within said
volume, wherein said second magnetic field is superimposed on said first
permanent magnetic field to provide said substantially uniform magnetic
field within said volume.
Description
FIELD OF THE INVENTION
This invention relates to permanent magnet assemblies for use in medical
applications and particularly to permanent magnet assemblies for use in
Magnetic Resonance Imaging (MRI) and/or Magnetic Resonance Therapy (MRT)
which produce a predetermined volume of substantially uniform magnetic
field extending in a first direction beyond the surface of the permanent
magnet assemblies.
BACKGROUND OF THE INVENTION
The principles of MRI are set forth in several patents such as U.S. Pat.
No. 5,304,933, which is incorporated herein by reference. MRT, sometimes
referred to as interventional MRI or intraoperative MRI, is the
performance of an interventional medical procedure on a patient in an MRI
system. During the procedure, a surgical instrument is inserted into a
patient in order to perform the procedure at a predetermined site in the
body. The MRT system is used in this case to monitor in quasi real-time
the correct placement of the instrument and also to observe the nature and
the extent of the effect of the intervention on the tissue.
In an MRI and/or MRT system a strong uniform magnetic field is required in
order to align an objects nuclear spins along the z-axis of a Cartesian
coordinate system having mutually orthogonal x-y-z areas. The required
strong uniform magnetic field, used for full body imaging, is normally in
the order of 0.1 to 2 Tesla. The imaging quality and the accuracy of an
MRI and/or MRT system is dependent on the degree of uniformity of the
strong uniform magnetic field. Uniformity is critical in MRI and/or MRT
applications because if the strong uniform magnetic field is not properly
uniform within the volume of interest, the desired discrimination between
different elements, due to the finely controlled magnetic field gradient,
will be subject to misinterpretation. Typically, the uniformity required
for the strong uniform magnetic field is within the order of 10 ppm within
the volume of interest. It is essential for MRT systems used in
interventional procedures to be based on an open structure, so as to
provide the physician easy access to the intervention site. Presently,
most MRI systems employ a large magnet, which effectively surrounds the
whole body of the patient, to produce the strong uniform magnetic field.
Such magnets are usually large superconductor resistive or permanent
magnets, each of which is expensive and heavy. Further, the access to the
patient in these cases is obstructed.
Attempts have been made to provide open magnets for interventional
procedures by employing two spaced-apart Helmholtz superconductive coil
assemblies. They provide only limited space between the assemblies
allowing for constricted access by only one person, such as a surgeon. See
U.S. Pat. No. 5,410,287 (Laskaris et al.) and U.S. Pat. No. 5,428,292
(Dorri et al.).
U.S. Pat. No. 4,875,485 (Matsutani) disclosed an apparently more compact
configuration, based on a pair of spaced-apart superconductive Helmholtz
coil assemblies, arranged for movement relative to a platform carrying the
patient. The access to the patient remains restricted in this case as
well, due to the additional space occupied by the cryostat. Also, the
movement of the coils independently of one another is impractical, because
the superconducting properties of the coils require extreme precision in
positioning of the two poles, in the absence of which the magnetic system
quenches.
In comparison to superconductive systems, permanent magnets are less
expensive, generate only a minimal unwanted fringe field and are not
involved with liquefied gas handling or vacuum requirements. Open access
MRI systems based on permanent magnets have been disclosed by U.S. Pat.
No. 4,829,252 (Kaufman) and U.S. Pat. No. 5,134,374 (Breneman). Both are
using a pair of opposing magnetic flat circular poles, employed one above
the other, with the patient lying down between the magnets. The poles are
mounted on end plates, supported by connecting members, which provide
return paths for the magnetic flux. These systems are massive and immobile
and the access to the patient is encumbered by the supporting structure.
A pair of opposing permanent magnet assemblies for use in MRI, each made of
concentric magnetic rings, composed of a set of magnetic polygonal blocks,
is disclosed in U.S. Pat. No. 5,332,971 (Aubert). Aubert teaches that the
opposing concentric rings within each of the pairs of permanent magnets
are to be spaced apart from each other the same distance. The magnet is
massive, weighing about 3 tons and is therefore not amenable to movement
relative to a patient's body.
In each of the above prior art magnets, used for providing the large
uniform magnetic field for MRI and/or MRT applications, the magnetic field
is generated in a first stage as uniformly as possible. More uniformity is
achieved subsequently by shimming.
Co-pending U.S. patent application Ser. No. 09/161,336, to Zuk et al.,
titled "MAGNETIC APPARATUS FOR MRI", filed Sep. 25, 1998, assigned to the
assignee of the present invention, the entire specification of which is
incorporated herein by reference disclosed, inter alia, magnetic apparatus
including an opposing pair of permanent magnetic assemblies defining an
open region therebetween in which an organ or body part is positioned for
imaging. The magnetic apparatus includes a plurality of gradient coils at
least one of which is positioned outside of the open region.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a permanent
magnet assembly for use in medical applications including MRI and/or MRT.
It is a further object of the present invention to provide a single annular
permanent magnet assembly for use in medical applications including MRI
and/or MRT applications which extends the volume of substantially uniform
magnetic field in a region adjacent to the surface of the permanent magnet
assembly.
It is a further object of the present invention to provide a single annular
permanent magnet assembly allowing free access to the volume of
substantially uniform magnetic field from the upper surface of the single
annular permanent assembly.
It is a further object of the present invention to provide a single annular
permanent magnet assembly, enabling insertion therethrough of a medical
instrument in a direction substantially parallel to the direction of the
uniform magnetic field.
It is a further object of the present invention to provide a first compact
annular permanent magnet assembly which is connected to a second compact
annular permanent magnet assembly through a connecting means for use in
medical applications including MRI and/or MRT which extend the
substantially uniform volume of magnet field in a region between the first
and second permanent magnet assemblies allowing lateral access around the
uniform volume.
It is a further object of the present invention to provide a permanent
magnet assembly which is compact, light, inexpensive and movable with
respect to a patient support.
It is yet a further object of the present invention to provide a method for
constructing an annular permanent magnet assembly for use in an MRI and/or
MRT device to produce a predetermined volume of substantially uniform
magnetic field extending in a first direction beyond an upper surface of
the permanent magnet assembly.
With these and other objects in view, the present invention contemplates a
permanent magnet assembly for use in an MRI and/or MRT device to produce a
predetermined volume of substantially uniform magnetic field extending in
a first direction beyond an upper surface of the permanent magnet
assembly. The permanent magnet assembly includes a first annular permanent
magnet having an upper and a lower surface, the upper surface of the first
annular permanent magnet being of a first magnetic polarity and the lower
surface of the first annular permanent magnet being of a second magnetic
polarity, the first annular permanent magnet having an inside diameter,
the first annular permanent magnet having at least a portion of the upper
surface of the first annular magnet lying in a first plane to provide a
first magnetic field in the predetermined volume, the first magnetic field
having a zero rate of change in the first direction perpendicular to the
first plane at a first point in the predetermined volume. The permanent
magnet assembly also including at least a second annular permanent magnet
having an upper and a lower surface, the upper surface of the second
annular permanent magnet being of the said first magnetic polarity and the
lower surface of the second annular permanent magnet being of the said
second magnetic polarity, the second annular permanent magnet having an
outside diameter which is smaller than the inside diameter of the first
annular permanent magnet, the second annular permanent magnet providing a
second magnetic field. The permanent magnet assembly finally including low
permeability material interconnecting the first annular permanent magnet
with the second annular permanent magnet, so that at least a portion of
the upper surface of the second annular permanent magnet is in a second
plane spaced from the first plane, whereby the second magnetic field is
superimposed on the first magnetic field, in the predetermined volume,
having a zero rate of change in the first direction at a second point
different from the first point. The low permeability material
interconnecting the first annular permanent magnet and the second annular
permanent magnet is preferably a high thermal conductivity material and is
slotted in order to reduce eddy currents.
In one embodiment of the invention, a permanent magnet assembly for use in
an MRI and/or MRT device is provided to produce a predetermined volume of
substantially uniform magnetic field extending in a first direction beyond
an upper surface of the permanent magnet assembly. The permanent magnet
assembly includes an annular permanent magnet having an upper and a lower
surface, the upper surface of the annular permanent magnet having a first
portion thereof lying in the first plane to provide a first magnetic field
in the predetermined volume, the first magnetic field having a zero rate
of change in the first direction perpendicular to the first plane at a
first point in the predetermined volume. The upper surface of the annular
permanent magnet has a second portion thereof lying in a second plane
forming a step and providing a second magnetic field in the predetermined
volume, the second magnetic field having a zero rate of change in the
first direction at a second point in the predetermined volume.
In another embodiment of the invention the upper surface of the first and
second annular permanent magnets each comprise a plurality of steps. Each
of the steps may be parallel to the first plane.
In another embodiment of the invention the upper surface of the first and
second annular permanent magnets are each inclined with respect to the
first plane.
In another embodiment of the invention the first and second annular
permanent magnets are movably mounted in their permanent magnet assembly,
their location along the axis of symmetry being determined by a set of
adjustment screws.
In still another embodiment of the invention a magnetic structure for use
in an MRI and/or MRT device is provided to produce a predetermined volume
of substantially uniform magnetic field in a region including a first
permanent magnet assembly having a first and a second surface and a second
permanent magnet assembly having a first and a second surface. The
structure further includes a mounting of low permeability material for
mounting the first permanent magnet assembly at a first location with the
first surface thereof facing one side of the region, and the second
permanent magnet assembly at a second location with the first surface
thereof facing the first surface of the first permanent magnet assembly on
an opposite side of the region so that the region is between the first
surfaces of the first and second permanent magnet assemblies, with lateral
free access from around. The first permanent magnet assembly has a first
annular permanent magnet with a first and a second surface. The first
surface of the first annular permanent magnet is of a first magnetic
polarity and the second surface of the first annular permanent magnet is
of a second magnetic polarity. The first annular permanent magnet has an
inside diameter. At least a portion of the first surface of the first
annular magnet lies in a first plane to provide a first magnetic field in
the region, the first magnetic field having a zero rate of change in a
first direction at a first point in the region. The first magnet assembly
also has at least a second annular magnet with a first and a second
surface. The first surface of the second annular permanent magnet is of
the first magnetic polarity and the second surface of the second annular
permanent magnet is of the the second magnetic polarity. The second
annular permanent magnet has an outside diameter which is smaller than the
inside diameter of the first annular permanent magnet, with at least a
portion of the first surface of the second annular magnet lying in a
second plane spaced from the first plane to provide a second magnetic
field whereby the second magnetic field is superimposed upon the first
magnetic field in the region, the second magnetic field having a zero rate
of change in the first direction at a second point different from the
first point. The second permanent magnet assembly has a third annular
permanent magnet with a first and a second surface. The first surface of
the third annular permanent magnet is of the second magnetic polarity and
the second surface of the third annular permanent magnet is of the first
magnetic polarity. The third annular permanent magnet has an inside
diameter. The third annular permanent magnet has at least a portion of the
first surface of the third annular magnet lying in a third plane to
provide a third magnetic field, whereby the third magnetic field is
superimposed upon the first and second magnetic fields in the region. The
third magnetic field has a zero rate of change in the first direction at a
third point different from the first and second points. The second magnet
assembly also has at least a fourth annular magnet having a first and a
second surface. The first surface of the fourth annular permanent magnet
is of the second magnetic polarity and the second surface of the fourth
annular permanent magnet is of the first magnetic polarity. The fourth
annular permanent magnet has an outside diameter which is smaller than the
inside diameter of the third annular permanent magnet, with at least a
portion of the first surface of the fourth annular permanent magnet lying
in a fourth plane spaced from the third plane to provide a fourth magnetic
field whereby the fourth magnetic field is superimposed on the first,
second and third magnetic fields in the region. The fourth magnetic field
has a zero rate of change in the first direction at, a fourth point
different from the first, second and third points.
In another embodiment of the invention, the first and second annular
permanent magnets are movably mounted in their respective permanent magnet
assembly, their location along the axis of symmetry being determined by a
set of adjustment screws. The third and fourth annular permanent magnets
are movably mounted in their respective permanent magnetic assembly, their
location along the axis of symmetry being determined by another set of
adjustment screws.
In another embodiment of the invention, the first and second permanent
magnet assemblies further include an outer casing capable of attachment to
the mounting.
In another embodiment of the invention the mounting includes a frame,
connected to a set of jaws by a movable screw. The movable screw may be
moved so as to rotate one or more of the jaws around an axis passing along
the screw, thereby allowing broader access to a patient's body part
positioned between the first and second permanent magnet assemblies.
In another embodiment of the invention the frame is controlled by a MRI
compatible motor so that the frame moves in a series of horizontal
positions, so that a composite image is formed.
In another embodiment of the invention the motor control of the frame
displaces the first and second permanent magnet assemblies in an axial
direction to bring the first and second permanent magnet assemblies either
closer together or farther apart from each other.
In accordance with the method of this invention a permanent magnet assembly
is constructed for use in an MRI and/or MRT device to produce a
predetermined volume of substantially uniform magnet field in a region.
The method includes selecting segments from a batch of equi-angular
segments, manufactured to have the same magnetization, so that variations
in a magnetic field of adjacent segments caused by inherent manufacturing
inaccuracies follow a cyclic curve having a regular period. The method
also includes combining the segments to form a first annular permanent
magnet. The first annular permanent magnet has a first and a second
surface thereof.
In another embodiment of the method, the method includes selecting segments
from a batch of equi-angular segments, manufactured to have the same
magnetization, so that variations in a magnetic field of adjacent segments
caused by inherent manufacturing inaccuracies follow a cyclic curve having
a regular period and combining the segments to form a second annular
permanent magnet. The second annular permanent magnet has a first and a
second surface thereof. The first and second annular magnets are then
interconnected with a low permeability material so as to form a first
permanent magnet assembly.
In another embodiment of the method, the method includes selecting segments
from a batch of equi-angular segments, manufactured to have the same
magnetization, so that variations in a magnetic field of adjacent segments
caused by inherent manufacturing inaccuracies follow a cyclic curve having
a regular period. The method also includes combining the segments to form
a third annular permanent magnet. The third annular permanent magnet has a
first and a second surface thereof.
In another embodiment of the method, the method includes selecting
equi-angular segments from a bath of segments, manufactured to have the
same magnetization, so that variations in a magnetic field of adjacent
segments caused by inherent manufacturing inaccuracies follow a cyclic
curve having a regular period and combining the segments to form a fourth
annular permanent magnet. The fourth annular permanent magnet has a first
and a second surface thereof. The third and fourth annular magnets are
then interconnected with a low permeability material so as to form a
second permanent magnet assembly.
In another embodiment of the method, the method includes forming a magnetic
structure by mounting the first permanent magnet assembly on a mounting of
low permeability material at a first location with the first surface
thereof facing one side of the region, and the second annular permanent
magnet at a second location with the first surface thereof facing the
first surface of the first annular permanent magnet on an opposite side of
the region. The region is between the first surfaces of the first and
second annular permanent magnets. The method still further includes
positioning the complementary annular permanent magnets so that the
respective cyclic curves are in anti-phase with each other, whereby the
variations in magnetic fields of adjacent segments from an average given
value cancel each other out to produce a substantially uniform magnetic
field in the region.
There is further provided, in accordance with a preferred embodiment of the
present invention an open magnetic apparatus for use in an MRI or IMRI
device to produce a predetermined volume of substantially uniform magnetic
field directed parallel to an axis of symmetry of the volume. The
apparatus includes a first electromagnet assembly disposed at a first
position along the axis, the first electromagnet assembly includes at
least a first electromagnet coil. The first coil is radially symmetric
with respect to the axis. The apparatus further includes a second
electromagnet assembly disposed at a second position spaced apart from the
first position of the electromagnet assembly along the axis. The second
electromagnet assembly includes at least a second electromagnet coil,
which is radially symmetric with respect to the axis. The first coil and
the second coil are equidistant from the center of the volume. The first
electromagnet assembly and the second electromagnetic assembly are adapted
for generating a first magnetic field within the volume. The apparatus
further includes a first permanent magnet assembly positioned at a third
position disposed between the first position and the second position along
the axis. The first permanent magnet assembly has an inner surface facing
the volume and an outer surface facing the first electromagnet assembly.
The first permanent magnet assembly includes a first permanent magnet
having an outer diameter. The first permanent magnet is radially symmetric
with respect to the axis and has a first magnetization direction parallel
to the axis. The first permanent magnet assembly further includes at least
a second annular permanent magnet coaxial with the first permanent magnet.
the second permanent magnet has an inner diameter larger than the outer
diameter of the first permanent magnet. The second permanent magnet is
radially symmetric with respect to the axis and has a second magnetization
direction parallel to the axis. The first permanent magnet assembly
further includes a first low magnetic permeability frame for supporting
the first permanent magnet and the second permanent magnet. The apparatus
further includes a second permanent magnet assembly opposed to the first
permanent magnet assembly. The second permanent magnet assembly is
positioned at a fourth position spaced apart from the third position along
the axis. The second permanent magnet assembly has an inner surface facing
the volume and an outer surface facing the second electromagnet assembly.
The second permanent magnet assembly includes a third permanent magnet
having an outer diameter. The third permanent magnet is radially symmetric
with respect to the axis and has a magnetization direction identical to
the first magnetization direction. The second permanent magnet assembly
further includes at least a fourth permanent magnet coaxial with the third
permanent magnet. The fourth permanent magnet has an inner diameter larger
than the outer diameter of the third permanent magnet. The fourth
permanent magnet is radially symmetric with respect to the axis and has a
magnetization direction identical to the second magnetization direction.
The second permanent magnet assembly further includes a second low
magnetic permeability frame for supporting the third permanent magnet and
the at least fourth permanent magnet. The inner surface of the first
permanent magnet assembly and the inner surface of the second permanent
magnet assembly define an open region therebetween. The volume is disposed
within the open region. The third position of the first permanent magnetic
assembly and the fourth position of second permanent magnetic assembly are
equidistant along the axis from the center of the volume. The first
permanent magnetic assembly and the second permanent magnetic assembly are
adapted to generate a second magnetic field superimposed on the first
magnetic field to provide the substantially uniform magnetic field within
the volume.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first magnetization direction is oriented parallel to the
second magnetization direction.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first magnetization direction is oriented anti-parallel to
the second magnetization direction.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first electromagnet coil and the second electromagnet coil
are operatively arranged as a Helmholtz pair.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first low magnetic permeability frame is adapted for
varying the position of at least the first permanent magnet along the
axis, and the second low magnetic permeability frame is adapted for
varying the position of at least the third permanent magnet along the
axis.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first low magnetic permeability frame is adapted for
varying the tilt angle of at least the first permanent magnet with respect
to the axis, and the second low magnetic permeability frame is adapted for
varying the tilt angle of at least the third permanent magnet with respect
the axis.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first electromagnet coil and the second electromagnet coil
are resistive electromagnet coils.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first electromagnet coil and the second electromagnet coil
are superconducting electromagnet coils.
Furthermore, in accordance with another preferred embodiment of the present
invention, the magnetic apparatus further includes a first gradient coil
assembly disposed between the outer surface of the first permanent magnet
assembly and the first electromagnet assembly, and a second gradient coil
assembly disposed between the outer surface of the second permanent magnet
assembly and the second electromagnet assembly.
Furthermore, in accordance with another preferred embodiment of the present
invention, each of the first gradient coil assembly and the second
gradient coil assembly includes one or more gradient coils selected from
an x-gradient coil, a y-gradient coil, a z-gradient coil and any
combination thereof.
Furthermore, in accordance with another preferred embodiment of the present
invention, each of the first gradient coil assembly and the second
gradient coil assembly is a multi-layer printed circuit assembly, and at
least one of the x-gradient coil, y-gradient coil and z-gradient coil of
each of the first gradient coil assembly and the second gradient coil
assembly is a substantially planar printed circuit coil.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first and the third permanent magnets are selected from a
disc-like permanent magnet having a circular cross section in a plane
perpendicular to the axis, a regular right polygonal prism-like permanent
magnet having a regular polygonal cross-section in a plane perpendicular
to the axis and having N sides, a ring-like annular permanent magnet and a
annular right regular polygonal permanent magnet having N sides.
Furthermore, in accordance with another preferred embodiment of the present
invention, N is equal to or larger than eight.
Furthermore, in accordance with another preferred embodiment of the present
invention, the second and the fourth permanent magnets are selected from a
ring-like annular permanent magnet and an annular right regular polygonal
permanent magnet having N sides.
Furthermore, in accordance with another preferred embodiment of the present
invention, N is equal to or larger than eight.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first electromagnet coil and the second electromagnet coil
are selected from a circular coil and a regular polygonal shaped coil
having N sides.
Furthermore, in accordance with another preferred embodiment of the present
invention, N is equal to or larger than eight.
Furthermore, in accordance with another preferred embodiment of the present
invention, the first permanent magnet and the third permanent magnet
include a plurality of permanently magnetized segments attached to
adjacent segments using a non-conductive adhesive, so as to reduce eddy
currents.
Furthermore, in accordance with another preferred embodiment of the present
invention, the segments are equi-angular segments.
Furthermore, in accordance with another preferred embodiment of the present
invention, the second annular permanent magnet and the fourth annular
permanent magnet include a plurality of permanently magnetized segments
attached to adjacent segments using a non-conductive adhesive, so as to
reduce eddy currents.
Furthermore, in accordance with another preferred embodiment of the present
invention, the segments are equi-angular segments.
There is further provided a method for constructing an open magnetic
apparatus for use in an MRI or IMRI device to produce a predetermined
volume of substantially uniform magnetic field directed parallel to an
axis of symmetry of the volume, the method includes the steps of providing
a first electromagnet assembly disposed at a first position along the
axis. The first electromagnet assembly includes at least a first
electromagnet coil, the first coil is radially symmetric with respect to
the axis. The method also includes the step of providing a second
electromagnet assembly disposed at a second position spaced apart from the
first position of the electromagnet assembly along the axis. The second
electromagnet assembly includes at least a second electromagnet coil. The
second coil is radially symmetric with respect to the axis. The first coil
and the second coil are equidistant from the center of the volume. The
first electromagnet assembly and the second electromagnetic assembly are
adapted for generating a first magnetic field within the volume. The
method also includes the step of providing a first permanent magnet
assembly positioned at a third position disposed between the first
position and the second position along the axis. The first permanent
magnet assembly has an inner surface facing the volume and an outer
surface facing the first electromagnet assembly. The first permanent
magnet assembly includes a first permanent magnet having an outer
diameter. The first permanent magnet is radially symmetric with respect to
the axis and has a first magnetization direction parallel to the axis. The
first permanent magnet assembly further includes at least a second annular
permanent magnet coaxial with the first permanent magnet. The second
permanent magnet has an inner diameter larger than the outer diameter of
the first permanent magnet. The second permanent magnet is radially
symmetric with respect to the axis and has a second magnetization
direction parallel to the axis. The first permanent magnet assembly also
includes a first low magnetic permeability frame for supporting the first
permanent magnet and the at least second permanent magnet. The method also
includes the step for providing a second permanent magnet assembly opposed
to the first permanent magnet assembly. The second permanent magnet
assembly is positioned at a fourth position spaced apart from the third
position along the axis. The second permanent magnet assembly has an inner
surface facing the volume and an outer surface facing the second
electromagnet assembly. The second permanent magnet assembly includes a
third permanent magnet having an outer diameter. The third permanent
magnet is radially symmetric with respect to the axis and has a
magnetization direction identical to the first magnetization direction.
The second permanent magnet assembly also includes at least a fourth
permanent magnet coaxial with the third permanent magnet. The fourth
permanent magnet has an inner diameter larger than the outer diameter of
the third permanent magnet. The fourth permanent magnet is radially
symmetric with respect to the axis and has a magnetization direction
identical to the second magnetization direction. The second permanent
magnet assembly also includes a second low magnetic permeability frame for
supporting the third permanent magnet and the fourth permanent magnet. The
inner surface of the first permanent magnet assembly and the inner surface
of the second permanent magnet assembly define an open region
therebetween. The volume is disposed within the open region. The third
position of the first permanent magnetic assembly and the fourth position
of second permanent magnetic assembly are equidistant along the axis from
the center of the volume. The first permanent magnetic assembly and the
second permanent magnetic assembly are adapted to generate a second
magnetic field superimposed on the first magnetic field to provide the
substantially uniform magnetic field within the volume.
Finally, there is also provided a method for providing a substantially
homogenous magnetic field within a volume disposed in an open region
within a magnetic apparatus. The method includes the steps of providing a
first electromagnet assembly disposed at a first position along the axis.
The first electromagnet assembly includes at least a first electromagnet
coil. The first coil is radially symmetric with respect to the axis. The
first step of providing also includes providing a second electromagnet
assembly disposed at a second position spaced apart from the first
position of the electromagnet assembly along the axis. The second
electromagnet assembly includes at least a second electromagnet coil. The
second coil is radially symmetric with respect to the axis. The first coil
and the second coil are equidistant from the center of the volume. The
method further includes the step of providing a first permanent magnet
assembly positioned at a third position disposed between the first
position and the second position along the axis. The first permanent
magnet assembly has an inner surface facing the volume and an outer
surface facing the first electromagnet assembly. The first permanent
magnet assembly includes a first permanent magnet having an outer
diameter. The first permanent magnet is radially symmetric with respect to
the axis and has a first magnetization direction parallel to the axis. The
first permanent magnet assembly also includes at least a second annular
permanent magnet coaxial with the first permanent magnet. The second
permanent magnet has an inner diameter larger than the outer diameter of
the first permanent magnet. The second permanent magnet is radially
symmetric with respect to the axis and has a second magnetization
direction parallel to the axis. The first permanent magnet assembly also
includes a first low magnetic permeability frame for supporting the first
permanent magnet and the second permanent magnet. The second step of
providing also includes providing a second permanent magnet assembly
opposed to the first permanent magnet assembly. The second permanent
magnet assembly is positioned at a fourth position spaced apart from the
third position along the axis. The second permanent magnet assembly has an
inner surface facing the volume and an outer surface facing the second
electromagnet assembly. The second permanent magnet assembly includes a
third permanent magnet having an outer diameter. The third permanent
magnet is radially symmetric with respect to the axis and has a
magnetization direction identical to the first magnetization direction.
The second permanent magnet assembly also includes at least a fourth
permanent magnet coaxial with the third permanent magnet. The fourth
permanent magnet has an inner diameter larger than the outer diameter of
the third permanent magnet. The fourth permanent magnet is radially
symmetric with respect to the axis and has a magnetization direction
identical to the second magnetization direction. The second permanent
magnet assembly also includes a second low magnetic permeability frame for
supporting the third permanent magnet and the at least fourth permanent
magnet. The inner surface of the first permanent magnet assembly and the
inner surface of the second permanent magnet assembly define an open
region therebetween. The volume is disposed within the open region. The
third position of the first permanent magnetic assembly and the fourth
position of second permanent magnetic assembly are equidistant along the
axis from the center of the volume. The first permanent magnetic assembly
and the second permanent magnetic assembly generate a first permanent
magnetic field in the volume. The method further includes the step of
electrically energizing the first electromagnet assembly and the second
electromagnetic assembly to provide a second magnetic field within the
volume, wherein the second magnetic field is superimposed on the first
permanent magnetic field to provide the substantially uniform magnetic
field within the volume.
DESCRIPTION OF THE DRAWINGS
In order to understand the invention and see how it may be carried out in
practice, several preferred embodiments will now be described, by way of
non-limiting example only, with reference to the accompanying drawings, in
which like components are designated by like reference numerals:
FIG. 1 is a pictorial perspective view of one segmented permanent magnet
assembly according to the invention;
FIG. 2 is a half cross-sectional view through the line I--I in FIG. 1;
FIG. 3 is a representation of the two dimensional distribution of the
magnetic field strength of the permanent magnet assembly of FIG. 1;
FIG. 4 is a pictorial representation of a first and a second permanent
magnet assembly;
FIG. 5 is a pictorial perspective view of the first and second permanent
magnet assemblies connected via a frame used for brain surgery;
FIG. 6 is a pictorial perspective view of the first and second permanent
magnet assemblies shown functionally in FIG. 5, used for performing
composite imaging;
FIG. 7 is a cross-sectional view through the first and second permanent
magnet assemblies and connecting means of FIG. 5;
FIG. 8 is a schematic representation of first and second permanent magnet
assemblies in which each permanent magnet assembly has five annular ring
magnets;
FIG. 9 is a graph showing the magnetic field strength as a function of
displacement along the z-axis between the first and second permanent
magnet assembly of FIG. 8;
FIG. 10 is a graphical representation showing the deviation from uniform
magnetic field along the z-axis between the first and second permanent
magnet assembly of FIG. 8;
FIG. 11 shows schematically a detail of the segmented construction of the
first and second permanent magnet assemblies, according to a preferred
embodiment of the invention;
FIGS. 12a and 12b show graphically a preferred mutual disposition of
opposing first and second permanent magnet assemblies having complementary
magnetic field variations;
FIG. 13 is a schematic diagram illustrating a front view of an open hybrid
magnetic apparatus for MRI/MRT including an electromagnet flux generator
and permanent magnet assemblies, in accordance with another preferred
embodiment of the present invention;
FIG. 14 is a cross sectional view of the hybrid magnetic apparatus of FIG.
13, taken along the lines XIV-XIV;
FIG. 15 is an isometric view illustrating the hybrid magnetic apparatus of
FIG. 13 attached to a motorized gantry with a head of a patient disposed
in the open region of the apparatus in a position suitable for imaging;
FIG. 16 is a part isometric, part cross-sectional exploded diagram
illustrating a part of one of the permanent magnet assemblies of FIG. 13
in detail;
FIG. 17 is a schematic graph illustrating the computed magnitude of the
magnetic field along the z-axis of an electromagnetic flux generator using
a Helmholtz pair electromagnet;
FIG. 18 is a schematic graph illustrating the computed magnitude of the
magnetic field along the z-axis common to a pair of permanent magnet
assemblies designed for generating a permanent magnetic field useful for
improving the magnetic field homogeneity of the magnetic field of the
Helmholtz pair electromagnets used in the computation illustrated in FIG.
17; and
FIG. 19 is a schematic graph illustrating the computed magnitude of the
magnetic field resulting from the superposition of the computed magnetic
fields illustrated in FIGS. 17 and 18.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is based, in part, upon the realization that whole
body imaging is not necessary for the performance of an interventional
medical procedure on a patient in an MRI system. It has been realized
that, in fact, a machine with a restricted field of view performs
satisfactorily in such a setting and can be built in a more efficient and
economical fashion than one built for accommodating a whole body.
Furthermore, in order to leave an open access to reach conveniently the
part oft he body on which the intervention is performed, the invention is
concerned with assemblies that are compact and also do not incorporate
ferromagnetic structures for the creation of return paths of the magnetic
flux.
In accordance with this invention, permanent magnet assemblies, each formed
from a plurality of annular concentric permanent magnets provide a volume
of substantially uniform magnetic field extending from a central portion
thereof.
The field strength of a single annular permanent magnet along a z-axis
perpendicular to its face and passing through its center is given by the
following expression, using the center of the permanent magnet as the
origin of the coordinate system:
##EQU1##
where:
.mu..sub.0 is the permeability of air
.mu. is the permeability of the annular permanent magnet
.PHI. is the magnetization
a is the inner radius of the annular permanent magnet
b is the outer radius of the annular permanent magnet
h is the height of the annular permanent magnet
The uniformity of the magnetic field in the volume is based on the fact
that any annular single permanent magnet has one point on its axis and
located outside its own plane, of maximum or minimum field strength, so
that the derivative of the field strength with respect to the z-axis there
is zero (i.e. dB/dz=0). It has been realized that by displacing the upper
surfaces of a plurality of concentric annular permanent magnets in the
assembly from each other, the respective points of zero derivative can be
displaced from each other, allowing the magnetic field in the volume to be
made uniform to within a defined tolerance of superimposing each of the
curves describing the field strength one on top of each other, so that the
point of zero derivative of one curve is superimposed on top of the
descending or ascending part of the other. In a like manner the upper
surfaces themselves can be created with steps to provide additional
displaced points of zero derivative.
The permanent magnet assemblies of this invention can be used in various
ways. One way of use is to construct a single permanent magnet assembly by
itself, to provide the uniform magnetic field adjacent to the upper
surface thereof.
FIGS. 1 and 2, taken together, show pictorially one embodiment of the
invention wherein a permanent magnet assembly 10 comprises inner and outer
aligned annular permanent magnets 11 and 12 formed of
Neodymium-Iron-Boron. The annular permanent magnets 11 and 12 are
preferably concentric. The inner annular permanent magnet 11 has a first
surface lying in a first plane and a second surface 14 lying in a
different plane, each plane being parallel to the x-y plane of the
permanent magnet assembly 10. The outer annular permanent magnet 12 has a
surface 15 lying in a second plane and a second surface 16 lying in a
different plane, each plane being parallel to the x-y plane of the
permanent magnet assembly 10. The inner and outer annular permanent
magnets 11 and 12 are interconnected by an intermediate annular band 17 of
low permeability material which holds them with their north and south
poles aligned in the same direction.
The complete structure comprising the inner annular permanent magnet 11,
the outer annular permanent magnet 12 and the intermediate annular band 17
are supported by a support annular band 20 formed of low permeability
material surrounding the outer annular permanent magnet 12. If desired,
the support annular band 20 may be integral with the intermediate annular
band 17, as shown in FIG. 2.
Referring particularly now to FIG. 2, a cross-section of the permanent
magnet assembly 10 taken through the line I--I in FIG. 1, the first
surface 15 of the outer annular permanent magnet is stepped such that a
periphery 21 of the outer annular permanent magnet 12 is higher than
successive intermediate portions 22, 23 and 24. Similarly, the first
surface 13 of the inner annular permanent magnet 11 has a periphery 25
higher than an intermediate portion 26 thereof. The permanent magnet
assembly 10 provides a volume 27 of substantially uniform magnetic field
which is adjacent to its upper surface. Uniformity of the magnetic field
in the volume 27 is based on the fact that any annular permanent magnet
has one point where the derivative of the field strength with respect to
the z-axis is zero (i.e. dB/dz=0), in a first direction perpendicular to
the face of the magnet. In order to achieve the desired uniformity in the
magnetic field of volume 27, the first surface 15 of the outer annular
permanent magnet 12 is provided with steps 21, 22, 23, 24 and the first
surface 13 of the inner annular permanent magnet 11 is provided with steps
25 and 26 constituting thereby a set of contiguous adjacent annular
permanent magnets. Thus, each step produces an additional displaced point
of zero derivative on the z-axis, riding on the ascending or descending
parts of the curves describing the field strength generated by other
steps. Consequently, the permanent magnet assembly 10 provides to the
volume 27 a plurality of points for which dB/dz=0, such that the volume 27
of the magnetic field is substantially uniform.
A circular bore 18, its axis constituting the z-axis of the permanent
magnet assembly 10, is formed in the inner annular permanent magnet 11 for
allowing access from below therethrough of a medical instrument and for
allowing an increased length of the medical instrument to protrude from
the first surface 13 of the inner annular permanent magnet 11, when the
permanent magnet assembly 10 is used in an MRT application. The circular
bore 18 is provided with a conical recess 33 in the second surface 14 of
the inner annular permanent magnet 11 of the permanent magnet assembly 10,
for partially accommodating the medical instrument. Complete free access
is allowed to the volume 27, when the volume is approached by the medical
instrument from above.
In another embodiment of the invention, the inner annular permanent magnet
11 has a series of continuous steps such that the steps take the form of
an incline. The incline is also possible on the steps of the outer annular
permanent magnet 12.
It has been found that a magnetic field with a uniformity of approximately
1000 ppm can be achieved prior to shimming, with a permanent magnet
assembly 10 as shown in FIGS. 1 and 2. FIG. 3 shows the uniformity (in
percentage) of the magnetic field generated by a 30 cm. diameter permanent
magnet assembly. The volume 27 in which the uniformity is 1250 ppm or less
is a cylinder adjacent to the upper face of the assembly 4.5 cm. in
height, with a diameter of 2 cm. The field strength is 785 Gauss.
However, the uniformity of magnetic field strength of the volume 27 can be
improved by means of shimming. There are standard shimming techniques,
well-known to those skilled in the art of magnet design, referred to as
passive shimming and active shimming.
Passive shimming can improve the magnetic field uniformity from orders of
approximately 1000 ppm to order of approximately 100 ppm. Active shimming
can improve the magnetic field uniformity from orders of approximately 100
ppm to orders of approximately 10 ppm and less.
Passive shimming is achieved by disposing shaped fragments 30 of magnetic
material of various polarities, of mumetal, or of soft iron on, for
example, the intermediate portion 26 of the inner annular permanent magnet
11 underneath a multi-layer printed circuit board 32.
Active shimming is achieved by printing shim coils 31 on several layers of
the separate layers of the multi-layer printed circuit board 32, the other
layers housing the gradient and RF coils, used ordinarily in MRI systems.
The multi-layer circuit board is seated in the recess 29, which is defined
by the area between the intermediate portion 23 of the outer annular
permanent magnet 12, the first surface 28 of the intermediate annular band
17 and above the first surface 13 of the inner annular permanent magnet
11. The multi-layer printed circuit board 32 is thus above the
intermediate portion 26 of inner annular permanent magnet 11 and does not
touch it. The uniformity of the magnetic field may be further improved by
disposing fragments 34 of magnetic material of various polarities, mumetal
or soft iron on, for example, the second surface 14 of the inner annular
permanent magnet 11.
In another embodiment of the invention not shown in the drawings, the
support annular band 20 and the intermediate annular band 17 are shaped so
as to allow the coaxial annular permanent magnets 11 an 12 to be finely
displaced and mutually offset along the common z-axis, so as to achieve
shimming. In this case, each of the coaxial annular permanent magnets 11
and 12 is connected to a low permeability lower plate via a plurality of
radially spaced-apart adjustment screws, attached to and cooperating with
the annular permanent magnets 11 and 12. Thus, the turning of the screws a
small amount in either clockwise or counter-clockwise direction moves the
corresponding annular permanent magnet (i.e. 11 or 12) toward or away from
the low permeability lower plate and consequently corrects the
non-uniformity in the volume 27 of uniform magnetic field to a desired
degree.
The permeability of the annular permanent magnets 11 and 12, is temperature
dependent so that temperature control can be a method of shimming. A
deviation of 1.degree. C. in the magnet temperature generates a change of
1000 ppm in the magnetic field strength. Each annular permanent magnet 11
and 12 has a temperature stabilization means for maintaining a
substantially constant temperature of the respective permanent magnet and
for varying it thereof for achieving shimming. The means consists of a
heater and of a feedback circuit which controls the temperature.
It will be appreciated that modifications to the basic structure of the
permanent magnet assembly 10 will be apparent to those skilled in the art,
without departing from the spirit of the invention. For example, it is
understood that other annular permanent magnet assemblies besides annular
permanent magnets 11 and 12 may be employed. Also the size of the annular
permanent magnets can vary according to the need.
Additional annular permanent magnets can be inserted between the inner and
outer annular permanent magnets 11 and 12, preferably such that an
intermediate support means of low permeability material is inserted
between each adjacent annular permanent magnet. However, in the extreme
embodiment where an external dimension of an internal annular permanent
magnet is equal to an internal dimension of an adjacent, external annular
permanent magnet, so that the two annular permanent magnets are
contiguous, the permanent magnet assembly 10 behaves as though the two
contiguous annular permanent magnets are a single structure. In either
case, the desired volume 27 of uniform magnetic field is still achieved.
A common problem with magnets is the generation of eddy currents. Eddy
currents are induced by momentarily changing the magnetic field as the
gradient field is formed. The eddy currents in turn produce a separate
magnetic field in the volume 27 of uniform magnetic field. In order to
reduce eddy currents, both the inner and outer annular permanent magnets
11 and 12 are formed of segments 19, each of which is permanently
magnetized in a known manner and then attached to a neighboring segment,
using a non-conductive glue.
Further, it is possible that local heating could be problematic, thus the
intermediate annular band 17 may be formed of high thermal conductivity
material so as to dissipate heat and reduce heat buildup. In an embodiment
where the intermediate annular band 17 is itself formed of electrically
conductive material, it too may be slotted radially so as to reduce eddy
currents.
Another way to use the permanent magnet assemblies is in opposed pairs, to
form the uniform magnetic field therebetween. FIG. 4 is a pictorial
representation of a set of first and second permanent magnet assemblies 40
and 42 each consisting of three concentric annular permanent magnets 42a,
42b, 42c and 40a, 40b, 40c (not shown in the drawing). Each permanent
magnet assembly is formed of segments 44, electrically insulated from a
neighboring segment so as to reduce eddy currents.
FIG. 5 shows pictorially details of the pair of permanent magnet assemblies
40 and 42 joined together via the frame 46 being shaped for imaging a
patient's brain 92, as manipulated by a plurality of surgeons 94 and 95
and a nurse 96. The pair of permanent magnet assemblies 40 and 42 joined
together via a frame 46 define a region having a volume 27 of
substantially uniform magnetic field, between the pair of permanent magnet
assemblies 40 and 42.
FIG. 6 is a pictorial side view of the pair of permanent magnet assemblies
40 and 42 connected via a frame 46, shown pictorially in FIG. 5, used for
performing composite imaging. The pair of permanent magnet assemblies 40
and 42 may be moved as a whole in the three directions x, y, and z by a
MRI compatible motor control unit 47, to shift the region of volume 27 of
uniform magnetic field and thus perform MRI and/or MRT on different
regions of the patient's brain 92. Thus, the volume 27 of uniform magnetic
field is shifted in relation to a patient placed between the pair of
permanent magnet assemblies 40 and 42. In use, the pair of permanent
magnet assemblies 40 and 42 connected via the frame 46 is placed in a
first position to produce a first image over a small field of view. The
pair of permanent magnet assemblies 40 and 42 connected via the frame 46
is then moved by the motor control unit 47, for example in the up and down
directions, so as to produce a series of spatially offset images. These
separate spatially offset images are then combined to form a composite
image, having a larger field of view.
FIG. 7 is a detailed cross-sectional view through the pair of permanent
magnet assemblies 40 and 42 along the line V--V in FIG. 5. The frame 46
comprises a set of two symmetrically mounted jaws 48 and 50 joined at an
end by a screw 52. An MRI compatible motor designated M is attached to the
screw 52 to provide displacement of each of the permanent magnet
assemblies 40 and 42 as a whole in an axial direction, to bring the
permanent magnet assemblies 40 and 42 either close together or further
apart, for shimming purposes.
Each of the permanent magnet assemblies 40 and 42 includes a plurality of
coaxial annular permanent magnets 40a, 40b, 40c and 42a, 42b, 42c which
are designed to provide the required volume 27 of uniform magnetic field
within a region 54 between the pair of permanent magnet assemblies 40 and
42. Each of the annular permanent magnets 40a, 40b, 40c and 42a, 42b, 42c
is enclosed in a low permeability material casing 67. It will be noted
that FIG. 7 shows only three coaxial annular permanent magnets, for the
sake of illustration and description.
Each of the coaxial annular permanent magnets 40a, 40b, 40c and 42a, 42b,
42c is coaxial with a common axis 56 of the corresponding pair or
permanent magnet assemblies 40 and 42, respectively. However, the coaxial
annular permanent magnets 40a, 40b, 40c and 42a, 42b, 42c themselves are
mutually offset along the common axis 56.
The contribution of each annular permanent magnet to the overall field
strength combines to generate a plurality of locations of zero derivative
in the z-direction allowing the magnetic field in the volume to be made
uniform to within a defined tolerance. The overall field strength along
the z-axis 56 of each permanent magnet assembly 40 and 42 is given by:
##EQU2##
where:
.DELTA..sub.zi =Z-z.sub.0i is the transverse separation, along the symmetry
axis 56, of z and z.sub.0i, a point located midway between the upper and
lower surfaces of the i.sup.th annular permanent magnet
.PHI. is the magnetization
.mu..sub.0 the permeability of air
.mu..sub.i is the permeability of the i.sup.th annular permanent magnet
.alpha..sub.i is the inner radius of the i.sup.th annular permanent magnet
b.sub.i is the outer radius of the i.sup.th annular permanent magnet
h.sub.i is the height of the i.sup.th annular magnet
the direction of the z axis for each permanent magnet assembly is towards
the volume 27 of uniform magnetic field. The overall field strength is a
superposition of the field strengths generated by each assembly.
Each of the coaxial annular permanent magnets 40a, 40b, 40c and 42a, 42b,
42c is fixed to an outer casing 60 via a plurality of radially spaced
apart set screws 62, attached to the magnets enclosures 67, cooperating
with the respective coaxial enclosures 67 of the annular permanent magnets
42a, 42b, and 42c, for achieving shimming of the permanent magnet assembly
42. It is apparent, as noted above, that the coaxial annular permanent
magnets 42a, 42b, 42c are mutually offset along the common axis 56 so as
to achieve shimming. Thus, turning of the set screws 62 a small amount in
either clockwise or counter-clockwise direction moves the corresponding
coaxial annular permanent magnet (i.e. 42a, 42b, 42c etc.) toward or away
from the outer casing 60 of the permanent magnet assembly 42 and
consequently corrects the non-uniformity in the region 54 of volume 27 of
uniform magnet field to a desired degree.
The free end of the jaws 48 and 50 is fixed to the outer casing 60 of the
permanent magnet assemblies 40 and 42 by means of a plurality of fixing
bolts 66. The whole structure 46 can be translated along the x, y and z
axis by the motor control unit 47 (not shown). Moreover, each of the jaws
48 and 50 may be rotated away from its opposing jaw by the motor control
unit 47, around an axis passing along screw 52, to allow the surgeon to
have complete free access to one side of the patient. The bolts 66 may
also be displaced, so that the respective pair of permanent magnet
assemblies 40 and 42 may be moved in the direction of arrows A and B and
thus accomplish shimming.
FIG. 8 is a schematic representation of an embodiment of the invention
including five coaxial annular permanent magnets 140a, 140b, 140c, 140d,
140e and 142a, 142b, 142c, 142d, 142e of the pair of permanent magnet
assemblies 140 and 142. The dimensions of the five coaxial annular
permanent magnets are shown in meters. The magnetic polarity of the
coaxial annular permanent magnets creates a volume 127 of homogenous
magnetic field.
Inasmuch as the pair of permanent magnet assemblies 140 and 142 are
identical in the embodiment thus far as described in FIG. 7, only one
permanent magnet assembly containing five coaxial annular permanent
magnets will be described in detail. However, it is understood that the
pair of permanent magnet assemblies 140 and 142 need not be identical.
Rather, the pair of permanent magnet assemblies 140 and 142 can have an
unequal number of annular permanent magnets.
Thus, in FIG. 8, the coaxial annular permanent magnets 140, 140b, 140c,
140d, 140e in the permanent magnet assembly 140 may be finely displaced
for shimming either towards or away from the complementary coaxial annular
permanent magnets 142a, 142b, 142c, 143d, and 142e in the opposing
permanent magnet assembly 142 along the common axis 156. An air gap of
approximately 5 millimeters is provided between the adjacent coaxial
annular permanent magnets 140a and 140b with an increased air gap of
approximately 10 millimeters provided between the adjacent coaxial annular
permanent magnets millimeters 140c and 140d. The remaining adjacent
coaxial annular permanent magnets 140b, 140c and 140d, 140e are
contiguous. Further, the overall average displacement between the pair of
permanent magnet assemblies 140 and 142 is approximately 25 cm. and their
approximately radius is 18 cm. The two opposing magnets weigh together 120
kg. The diameter of the spherical volume 127 of uniform magnetic field is
16 cm.
FIG. 9 is a graph showing magnetic field strength as a function of
displacement along the z-axis, at a given value of y. It is seen that the
field strengths of the opposing permanent magnet assemblies 40 and 42
superimpose so as to form a region 54 of a volume 27 of substantially
homogenous magnet field having a magnitude of approximately 1000 Gauss.
FIG. 10 is a graphical representation of the magnetic field in volume 27
along the z-axis, showing the uniformity in ppm. The effect of
superposition of curves having spaced apart maxima is illustrated.
As noted above, a particular design features of the permanent magnet
assembly 10 is the ease with which shimming may be used to achieve a
volume 27 of very high magnetic field uniformity typically to within
several ppm.
FIG. 11 shows schematically a detail of the construction of the coaxial
annular permanent magnet 40a. Each coaxial annular permanent magnet
comprises eighteen permanently magnetized segments 70 to 87 which are
supplied in batches and are normally guaranteed by the manufacturer to
have a peak to peak variation in magnetic field of 1%. The segments 70 to
87 each subtend an angle of 200 at the center of the coaxial annular
permanent magnet 40a and are joined by an electrically non-conductive
adhesive so as to reduce eddy currents, as explained above.
Owing to the slight difference in magnetic field between different segments
70 to 87 in each batch, it is often difficult to achieve a volume 27 of
even a coarse magnetic field uniformity in the region 54 between a pair of
opposing permanent magnet assemblies 40 and 42. It will be understood that
at least a coarse magnetic field uniformity is a prerequisite to the fine
tuning achieved using passive and active shimming.
FIGS. 12a and 12b show graphically how such a volume 27 of coarse magnetic
field uniformity is achieved, notwithstanding the inherent variation in
magnetic field of different segments 70 to 87. The magnetic field of
different segments 70 to 87 is measured and adjacent segments are selected
from the batch having slightly different field strengths so as to follow a
substantially cyclic curve 90. Thus, as shown in FIGS. 11 and 12a, the
particular segments 70 and 80 have a minimum magnetic field as compared to
segment 85 which has a maximum magnetic field. The segments from 70 to 75
have increasing magnetic fields following the cyclic curve 90 in contrast
to segments from 75 to 80 which have decreasing magnetic fields following
the cyclic curve 90. Each of the pair of permanent magnet assemblies 40
and 42 is constructed according to this approach and are then opposed to
one another in anti-phase such that the relationship of the corresponding
magnetic fields of the pair of permanent magnet assemblies 40 and 42
corresponds to the two anti-phase cyclic curves shown in FIGS. 12a and
12b. The variations in magnetic field from its average value as described
by lines 88 and 89 along the two cyclic curves then exactly cancel each
other out, such that a region 54 between the pair of permanent magnet
assemblies 40 and 42 has a volume 27 of uniform magnetic field.
It will be appreciated that, due the limited magnetic field magnitude
achievable by using commercially available permanent magnets (such as, for
example, Neodymium-Iron-Boron magnets), the magnitude of the substantially
uniform magnetic field which is typically achievable within the imaging
volume of such permanent magnetic probes is within the range of
approximately 0.1-0.3 Tesla (1000-3000 Gauss). However, in many cases it
is desirable to increase the magnitude of the magnetic field within the
imaging volume. Increasing the magnitude of the magnitude field improves
the signal to noise ratio (S/N) which improves image resolution and
enables reducing the time required for image acquisition. The latter may
be particularly advantageous for inter-operative MRI in which it is
desirable to minimize the time required for image acquisition during the
performance of surgery on a patient.
Trying to increase the magnitude of the magnetic field within the imaging
volume by designing larger permanent magnet assemblies has practical
limits since increasing the size of the annular permanent magnets is
relatively inefficient because the intensity of the magnetic field is
inversely proportional to the third power of the distance of the magnetic
material from the imaging volume. Therefore, adding additional annular
permanent magnets of larger diameters has a practical limit due to the
prohibitive cost of the magnetic material. Additionally, increasing the
diameter of the permanent magnet assemblies beyond a certain size is not
desirable because it may limit access to the imaged organ or body part
during surgery.
Reference is now made to FIGS. 13-15. FIG. 13 is a schematic diagram
illustrating a front view of an open hybrid magnetic apparatus for MRI/MRT
including an electromagnet flux generator and permanent magnet assemblies,
in accordance with another preferred embodiment of the present invention.
FIG. 14 is a cross sectional view of the hybrid magnetic apparatus of FIG.
13, taken along the lines XIV--XIV. FIG. 15 is an isometric view
illustrating the hybrid magnetic apparatus of FIG. 13 attached to a
motorized gantry with a head of a patient disposed in the open region of
the apparatus in a position suitable for imaging.
The magnetic apparatus 150 of FIG. 13 includes an electromagnetic flux
generator including two opposing electromagnet assemblies 150 and 152. The
electromagnet assemblies 150 and 152 include housings 150H and 152H,
respectively. The housings 150H and 152H are attached to a base member
156. The electromagnet assemblies 150 and 152 are generally cylindrically
shaped and are spaced apart from each other. However, the electromagnet
assemblies 150 and 152 may have any other suitable shape depending, inter
alia, on the type and detailed design of the electromagnet assemblies
used, the type and arrangement of electromagnet coils (not shown) included
in the electromagnet assemblies 150 and 152 and on the detailed design of
the housings 150H and 152H as disclosed in detail hereafter. The housing
150H has an internal, surface 150A and the housing 152H has an internal
surface 152A. The internal surface 150A faces the internal surface 152A.
The magnetic apparatus 150 further includes two opposing permanent magnet
assemblies 160 and 162. The permanent magnet assemblies 160 and 162 are
attached to the base 156 by two mounting members 176 and 180,
respectively. The permanent magnet assembly 160 has an inner surface 160A
and an outer surface 160B. The permanent magnet assembly 162 has an inner
surface 162A and an outer surface 162B. The inner surfaces 160A and 162A
define an open region 164 therebetween, an imaging volume 177 is
positioned within the open region 164. The dashed line 175 represents the
line of intersection of a mid-plane (not shown) with the plane in which
the cross section of FIG. 13 lies (represented by the plane of the page of
the drawing of FIG. 13). The mid-plane is equidistant from the surfaces
160A and 162A. The mid-plane is also equidistant from the surfaces 150A
and 152A of the housings 150H and 152H, respectively.
The magnetic apparatus 150 further includes two opposing gradient coil
assemblies 190 and 192. The gradient coil assembly 190 is disposed between
the surface 150A of the housing 150H and the outer surface 160B of the
permanent magnet assembly 160. The gradient coil assembly 192 is disposed
between the surface 152A of the housing 152H and the outer surface 162B of
the permanent magnet assembly 162. Each of the gradient coil assemblies
190 and 192 may include an x-gradient coil a y-gradient coil and a
z-gradient coil and may be constructed similar to the multi-layer printed
circuit assemblies disclosed in detail by Zuk et al. and illustrated in
FIGS. 5-8 of U.S. patent application Ser. No. 09/161,336. The gradient
coil assemblies 190 and 192 are disposed outside of the open region 164
which increases the space available within the open region 164 as
disclosed in detail in U.S. patent application Ser. No. 09/161,336.
It is noted that, the gradient coil assemblys' structure and positioning
may be any suitable structure and positioning which is known in the art
including, but not limited to, all the different structures and
arrangements of gradient coils or gradient coil combinations (or gradient
coil pair arrangements) disclosed in detail in U.S. patent application
Ser. No. 09/161,336 to Zuk et al.
Similarly, the transmitting and receiving RF coils (not shown) which are
used for imaging with the magnetic apparatus 150 of the present invention,
may have any suitable known structure and spatial position which is known
in the art of RF coils including, but not limited to, all the different
structures and arrangement of RF coils disclosed in detail in U.S. patent
application Ser. No. 09/161,336 to Zuk et al.
Each of the permanent magnet assemblies 160 and 162 includes a plurality of
generally annular or disc-like permanent magnets (not shown in FIG. 13 for
the sake of clarity of illustration). The line 188 of FIG. 13 represents
the common axis passing through the geometrical center of all of the
annular permanent magnets of the permanent magnet assemblies 160 and 162
(and through the geometrical center of the disc-like permanent magnets if
they are included in the permanent magnet assemblies 160 and 162) as
disclosed in detail hereinbelow (see FIG. 16). The axis 188 is also
defined as the z-axis of the hybrid magnetic apparatus 150. The main
magnetic field generated by the magnetic apparatus 150 within the imaging
volume 177 is directed in parallel to the z-axis 188.
The structure and arrangement of the permanent magnets composing the
permanent magnet assemblies 160 and 162 has been disclosed in detail
hereinabove and in U.S. patent application Ser. No. 09/161,336, to Zuk et
al. In accordance with a preferred embodiment of the present invention,
the shape and arrangement of the annular permanent magnets of permanent
magnet assembly 160 may be similar to the shape and arrangement of the
annular permanent magnets 40a, 40b and 40c of permanent magnet assembly 40
of FIGS. 4-7 and the shape and arrangement of the annular permanent
magnets of permanent magnet assembly 162 may be similar to the arrangement
of the annular permanent magnets 42a, 42b and 42c of permanent magnet
assembly 42 of FIGS. 4-7.
In accordance with another preferred embodiment of the present invention,
the shape and arrangement of the annular permanent magnets of permanent
magnet assembly 160 may be similar to the shape and arrangement of the
annular permanent magnets 140a, 140b and 140c of permanent magnet assembly
140 of FIG. 8 and the shape and arrangement of the annular permanent
magnets of permanent magnet assembly 162 may be similar to the arrangement
of the annular permanent magnets 142a, 142b and 142c of permanent magnet
assembly 142 of FIG. 8.
In accordance with yet another preferred embodiment of the present
invention, each of the permanent magnet assemblies 160 and 162 may be
constructed similar to the permanent magnet assembly 10 of FIGS. 1-2, with
the proviso that the permanent magnet assemblies 160 and 162 are arranged
within the apparatus 150 such that the permanent magnet assembly 160 is
oriented as a mirror image of the permanent magnet assembly 162 with
respect to the mid-plane (not shown) equidistant from the surfaces 150A
and 152A of the electromagnet assemblies 150 and 152, respectively.
However, The orientation of magnetization of the two annular permanent
magnets included within any of the complementary opposing pairs of the
annular permanent magnets of the permanent magnet assemblies 160 and 162
are identical, while different pairs of opposing complementary annular
permanent magnets may have their magnetization directions oriented
parallel or anti parallel to each other, depending on the specific design
of the magnetic assemblies as disclosed in detail hereinabove and
illustrated in FIG. 8. And as disclosed in detail by Zuk et al. in U.S.
patent application Ser. No. 09/161,336.
In accordance with yet another preferred embodiment of the present
invention, the permanent magnet assemblies 160 and 162 may be constructed
similar to the permanent magnet assemblies disclosed in U.S. patent
application Ser. No. 09/161,336, to Zuk et al.
Reference is now made to FIG. 16 which is a part isometric, part
cross-sectional exploded diagram illustrating a part of the permanent
magnet assembly 160 of FIG. 13 in detail. The magnet assembly 160 includes
a first disc-like permanent magnet 200 having an inner surface 204B and an
outer surface 204C, and a second annular permanent magnet 204 having an
inner surface 200B and an outer surface 200C. The z-axis 188 passes
through the geometrical center of the permanent magnets 200 and 204 which
are concentrically arranged with respect to the z-axis 188. The surfaces
200B and 200C are parallel surfaces and are substantially perpendicular to
the z-axis 188. The surfaces 204B and 204C are parallel surfaces and are
also substantially perpendicular to the z-axis 188. The magnet assembly
160 also includes two magnet protecting members 206 and 208. The magnet
protecting members 206 and 208 are attached or glued to the first
permanent magnet 200 and are preferably made from a non-ferromagnetic, and
preferably non-electrically conducting material such as plastic,
fiberglass or the like, but other suitable materials may also be used. The
magnet protecting members 206 and 208 have annular notches 206A and 208A,
respectively, therewithin. The magnet protecting member 206 has a
plurality of rectangular notches 210 therein which are distributed along
the inner circumference 206B thereof.
The magnet assembly 160 also includes an outer supporting ring-like member
212 and an inner ring-like supporting member 216. The supporting members
212 and 216 are preferably made from non-ferromagnetic, electrically
non-conducting material such as, plastic, fiberglass or the like, but
other suitable materials may also be used. The outer supporting member 212
is attached to the annular permanent magnet 204 by a suitable glue or by
other suitable attaching means known in the art. The outer supporting
member 212 has a plurality of circumferential threaded holes 213
therewithin. The holes 213 are adapted to receive a plurality of retaining
screws 215 therein. The outer supporting member 212 has a second plurality
of threaded holes 217 therewithin. The threaded holes 217 are adapted to
receive a plurality of upper offsetting screws 219 therewithin.
The magnet assembly 160 further includes a plurality of L-shaped members
214 and 214A. The L-shaped members 214 and 214A are disposed between. A
portion of each one of the L-shaped members 214 and 214A is disposed
between the outer supporting member 212 and the inner supporting member
216. The retaining screws 215 are used to firmly attach the L-shaped
members 214 and 214A to the outer and inner supporting members 212 and
216, respectively by screwing them into the threaded holes 213 and into
suitable threaded holes 222 passing within the L-shaped members 214 and
214A, and through suitable threaded holes 223 within the inner supporting
member 216.
Another different portion of each one of the L-shaped members 214 and 214A
is disposed between the inner surface 204A of the annular permanent magnet
204 and the magnet protecting members 206 and 208. The L-shaped members
214 and 214A are preferably made from a non-ferromagnetic electrically
non-conducting material such as, plastic, fiberglass or the like, but
other suitable materials may also be used. Each of the of L-shaped members
214A has a threaded hole 215B therein. Each of the threaded holes 214B is
adapted to receive a lower offsetting screw 220 therein. In a preferred
embodiment of the present invention, the inner supporting ring 216 has
four threaded holes 217 therein, and the number of the L-shaped members
214A is four, but other different numbers of holes 217 and of L-shaped
members 214A may also be used.
The upper offsetting screws 219 are screwed into the threaded holes 217
until they are in contact with the surface of the magnet protecting member
206 within the annular notch 206A. The lower offsetting screws 220 are
screwed into the threaded holes 214B of the L-shaped members 214A until
they are in contact with the surface of the magnet protecting member 208
within the annular notch 208A. By suitably adjusting the position of one
or more of the upper offsetting screws 219 and/or the lower adjusting
screws 220, the position of the inner disc-like permanent magnet 200 along
the z-axis may be offset in a direction towards or away from the inner
supporting member 216. This serves to fine-tune the homogeneity of the
main magnetic field within the imaging volume 177 as is disclosed in
detail hereinafter.
It is noted that, suitable adjustment of one or more of the upper
offsetting screws 219 and/or the lower offsetting screws 220 may also be
used to tilt the surfaces 200B and 200C with respect to the Z-axis 188,
such that the surface 200B of the disc-like permanent magnet 200 is tilted
at an angle, referred to as the tilt angle hereinafter (not shown), with
respect to the surface 204B of the annular permanent magnet 204. Such
tilting may also be used for fine tuning the homogeneity of the main
magnetic field within the imaging volume 177.
Additionally, in accordance with other preferred embodiments of the present
invention, the position and the tilt angle of the annular permanent magnet
204 with respect to the z-axis 188 may also be varied by using a similar
tilting and/or position adjusting mechanism (not shown) for adjusting the
tilt angle of the annular permanent magnet 204 with respect to the z-axis
188. Such a mechanism may be constructed by using adjusting screws (not
shown) for changing the tilt and position of the annular permanent magnet
204 relative to an external housing or frame (not shown) within which the
permanent magnet assembly is mounted.
It will be appreciated by those skilled in the art that, in permanent
magnet assemblies having more than two permanent magnets, the position
and/or tilt adjustment mechanisms may be adapted to enable individual
adjustment of the position and/or the tilt angle of one or more of the
permanent magnets of the permanent magnet assemblies. Such adaptations to
the adjustment machanisms are well known in the art and are not be
disclosed in detail herein.
It is further noted that, the disc-like permanent magnet 200 may also be
rotated around the z-axis 188 with respect to the annular permanent magnet
204. The rotation is performed by loosening the offsetting screws 219
and/or 220 and rotating the disc-like permanent magnet 200 and the magnet
protecting members 206 and 208 attached thereto around the z-axis 188. The
rotation may be performed by inserting into some of the rectangular
notches 210 prongs (not shown) of an adapted turning tool (not shown) and
rotating the tool. After the rotation is performed, the offsetting screws
219 and 220 may be readjusted and the offsetting or tilting procedures
disclosed hereinabove may then be performed if further fine tuning is
required.
It is yet further noted that, the permanent magnet assembly 160 of FIG. 16
may also include an external housing (not shown) preferably made from a
non-ferromagnetic material having low electrical conductivity such as
plastic, fiberglass or the like. This housing may be used for protecting
the permanent magnets 204 and 206 and for attaching the permanent magnet
assemblies 160 and 162 to the mounting members 176 and 180 of FIG. 13.
It is still further noted that, while the permanent magnet 200 is shown as
a disc-like permanent magnet, in may also be an annular permanent magnet
having a hole (not shown) therewithin, such as, for example, the annular
permanent magnet 11 of FIG. 1. Additionally, while the permanent magnet
assembly 160 of FIG. 16 is shown to have two permanent magnets, other
preferred embodiments may have other larger numbers of concentric
permanent magnets. Typically, the permanent magnet assemblies 160 and 162
may each include 2-6 concentric permanent magnets as disclosed hereinabove
and in U.S. patent application Ser. No. 09/161,336, to Zuk et al.
Furthermore, it will be appreciated by those skilled in the art that, the
particular mechanical design of the parts for holding together and turning
the magnetic assemblies of FIG. 16 are given by way of example only and
that many other arrangements for adjusting the position and the tilt angle
of the permanent magnets with respect to the z-axis may be used which are
within the scope and spirit of the present invention.
Briefly returning to FIG. 15, The magnetic apparatus 150 is shown attached
to a motorized gantry 199. The base 156 is rigidly attached to the gantry
199. A patient is shown lying on a surgical table 181 and the head of the
patient 179 is disposed in the region 164 between the permanent magnet
assemblies 160 (not shown) and 162. In this position, part of the brain
(not shown) of the patient may be placed within the imaging volume 177
(not shown for the sake of clarity of illustration) for imaging thereof.
The details of the gantry 199 are not the subject of the present invention
and are not disclosed hereinafter.
Turning back to FIG. 13, the electromagnet assemblies 150 and 152 may be
any suitable type of electromagnet known in the art. In accordance with
one preferred embodiment of the present invention the electromagnet
assemblies 150 and 152 may be resistive electromagnets each including one
or more resistive coils for generating a magnetic field in the imaging
volume 177. The structure and design of resistive electromagnets is well
known in the art and will not be disclosed in detail hereinafter.
Preferably, the resistive electromagnets 150 and 152 may be implemented
using a of Helmholtz coil pair (not shown). One of the coils of the
Helmholtz pair (not shown) is disposed within the housing 150H of the
electromagnet assembly 150 and the other coil (not shown) of the Helmholtz
pair is disposed within the housing 152H of the electromagnet assembly
152. The Helmholtz coil pair is arranged such that the z-axis passes
through the center of each of the coils (not shown) of the Helmholtz pair.
The structure and design considerations of Helmholtz pair electromagnets
is well known in the art and will not be disclosed in detail hereinafter.
Briefly, the structure and geometrical parameters of Helmoltz coil pair
are defined in page 919 of "THE McGRAW-HILL DICTIONARY OF SCIENTIFIC AND
TECHNICAL TERMS" Fifth edition, Edited by Sybil P. Parker (1994). An
advantage of the Helmholtz pair electromagnet is that it is relatively
simple and inexpensive to implement while providing a fairly uniform
magnetic field which may be further corrected and turned by the permanent
magnet assemblies 160 and 162.
Reference is now made to FIG. 17 which is a schematic graph illustrating
the computed magnitude of the magnetic field along the z-axis of an
electromagnetic flux generator using Helmholtz pair electromagnets. The
vertical axis of the graph represent the magnitude of the magnetic field
generated by a helmholtz pair electromagnet along the z-axis which is the
common axis passing through the center of both of the coils (not shown) of
the Helmholtz pair electromagnet. The horizontal axis represents the
distance in centimeters along the z-axis of the magnetic flux generator.
The zero point on the horizontal axis represents the midpoint of the
z-axis, negative distance represent points on the z-axis left of the zero
point and positive distance values represent points on the z-axis right of
the zero point. The computation was performed for a Helmholtz pair having
a coil radius of approximately 50 centimeter. The distance between the
centers of the coils (not shown) of the Helmholtz pair is therefore
approximately 50 centimeters. The curve 230 represents the computed
magnitude of the magnetic field at the z-axis as a function of the
position along the z-axis 188. The graph shows a magnetic field magnitude
of approximately 4316 Gauss at the zero point of the z-axis. At the
distances of +5.0 centimeters and -5.0 centimeters on the z-axis the
computed magnitude of the magnetic field drops to a value of approximately
4315.5 Gauss which is a drop of 0.5 Gauss for a 5 centimeter distance from
the zero point. By itself, this field homogeneity is not adequate for
conventional MRI imaging.
Reference is now made to FIG. 18 which is a schematic graph illustrating
the computed magnitude of the magnetic field along the z-axis common to a
pair of permanent magnet assemblies designed for generating a permanent
magnetic field useful for improving the magnetic field homogeneity of the
magnetic field of the Helmholtz pair electromagnets whose magnetic field
is computed in FIG. 17. The vertical axis of the graph represent the
computed magnitude of the magnetic field generated the pair of permanent
magnet assemblies along the z-axis which is the common axis passing
through the center of both permanent magnet assemblies (not shown). The
horizontal axis represents the distance in centimeters along the z-axis
common to both of the permanent magnet assemblies. Each of the two
permanent Magnet assemblies for which the computation was performed
included three concentric annular permanent magnets (not shown). The outer
diameter of the largest outermost annular permanent magnet of each of the
permanent magnet assemblies was approximately 19 centimeters.
The zero point on the horizontal axis represents the midpoint of the
z-axis, negative distance represent points on the z-axis left of the zero
point and positive distance values represent points on the z-axis right of
the zero point. The curve 232 represents the computed magnitude of the
magnetic field at the z-axis as a function of the position along the
z-axis 188. The computation shows a magnetic field magnitude of
approximately 1000 Gauss at the zero point of the z-axis. At the distances
of +5.0 centimeters and -5.0 centimeters on the z-axis the magnitude of
the magnetic field rises to a value of approximately 1000.5 Gauss which is
an increase of 0.5 Gauss for a 5 centimeter distance from the zero point.
Reference is now made to FIG. 19 which is a schematic graph illustrating
the computed magnitude of the magnetic field resulting from the
superposition of the computed magnetic fields illustrated in FIGS. 17 and
18. The vertical axis of the graph of FIG. 19 represent the magnitude at
the z-axis of the superimposed computed magnetic fields represented by the
curves 230 and 232 of FIGS. 17 and 18, respectively. The horizontal axis
represents the distance in centimeters along the z-axis common to the
permanent magnet assemblies and to the Helmholz pair electromagnet.
The curve 234 represents the computed magnitude at the z-axis of the
superimposed magnetic fields represented by the curves 230 and 232 as a
function of the position along the z-axis 188. The computated curve 234
shows a mean magnetic field magnitude along the z-axis of approximately
5316 Gauss with a homogeneity of approximately .+-.10 parts per million
(ppm) within a distance of approximately .+-.7 centimeters from the zero
point of the z-axis.
It is noted that, the computed superimposed magnetic field magnitude
represented by the curve 234 exhibits a very steep rate of rise at
distances of approximately 7 centimeters or higher from the zero point of
the z-axis. These "shoulders" with steep rate of rise of the curve 234 are
labeled 234A and 234B. This feature is advantageous since it prevents
magnetic field folding during imaging when the linear magnetic field
gradients generated by the gradient coil assemblies 190 and 192 of FIG. 13
are superimposed on the main magnetic field of the hybrid magnetic
apparatus 150. Magnetic field folding typically results in deterioration
of image quality and resolution induced by the defocusing effects of RF
signals which are generated in tissue regions outside of the FOV but
adjacent thereto. Such regions may experience magnetic field values which
are identical to the values of the magnetic field experienced by tissue
regions within the field of view. Therefore, the RF signals generated by
tissue regions inside and outside the FOV will have identical frequencies
which results in pixel folding and defocusing of the image within the FOV.
This problem is particularly aggravated in imaging situations, such as,
for example, in restricted organ or body part imaging in which the local
receiving RF coils used may be positioned close to tissue regions disposed
outside of the FOV.
Therefore, the steep rate of change of the magnetic field at the curve
shoulders 234A and 3234B advantageously results in an improved focused
field of view (FOV) due to elimination or reduction of pixel folding.
Another advantage of the main magnetic fields generated by the hybrid
magnetic apparatus of the present invention is that the magnitude of the
main magnetic field is substantially higher than the magnetic field
achieved by using only the permanent magnet assemblies disclosed
hereinabove, while still having an acceptable degree of homogeneity. This
advantage results from the method of designing of the hybrid magnetic
apparatus 150 of the present invention. In the design method of the
present invention the electromagnet coils of the electromagnet assemblies
150 and 152 and the permanent magnet assemblies 160 and 162 are designed
together such that the permanent magnet assemblies 160 and 162 are adapted
to improve the homogeneity of the overall resulting magnetic field and to
provide a steep rate of change of the magnetic field magnitude at the
edges of the FOV.
It is noted that, while the particular computed example of the magnetic
field curve 234 includes shoulders 234A and 234B having a steep rate of
rise of the magnetic field amplitude, the present invention may also be
practiced by generating magnetic field curves having shoulder curve
portions with a steep rate of decrease of magnetic field magnitude. Thus,
the curve shoulders of the magnetic field curves useful in the present
invention are generally characterized by a steep rate of change,
irrespective of the change direction.
It is further noted that, in accordance with another preferred embodiment
of the present invention, the electromagnet assemblies 150 and 152 may be
superconducting electromagnet assemblies. The design and operation of
superconducting magnets is well known in the art and will not be disclosed
in detail hereinafter. Any suitable design of superconducting
electromagnets known in the art may be adapted for implementing the hybrid
magnetic apparatus 150 of the present invention. For example, the
electromagnet assemblies 150 and 152 may be implemented as superconducting
magnets having a single or multiple superconducting coils. The
electromagnet assemblies 150 and 152 may also be implemented as
superconducting magnets configured as a superconducting Helmholtz pair.
It is still further noted that if the electromagnets used in the
construction of the electromagnet assemblies 150 and 152 are resistive
electromagnets, any suitable coil configuration known in the art may be
used, including, but not limited to, designs having multiple electromagnet
coils which are disposed within each of the electromagnet assemblies 150
and 152.
It will be appreciated that the MRI and/or interventional magnetic
resonance imaging (IMRI) systems (not shown) which include the hybrid
magnetic apparatus include additional parts and components which are
necessary to the operation of such systems for imaging. Such components
may include power supplies, receiving and transmitting RF coils and RF
signal generator and receiver circuitry, active shimming coils, passive
shimming members, gradient amplifiers, and various components for
controlling the operation of the system for acquiring and processing
imaging data and for displaying and storing MRI images. Additionally, the
systems may include temperature stabilization systems for regulating the
temperature of various system components such as the permanent magnet
assemblies, the electromagnet assemblies. Furthermore, if the
electromagnet assemblies 150 and 152 are of the superconducting
electromagnet type, the system may include all the necessary components
such as cryogenic Dewars and the like for cooling the superconducting
coils to the required temperature and for maintaining the required
temperature. The construction and operation of such components is well
known in the art and is therefore not disclosed herein in detail.
It will be appreciated by those skilled in the art that the number, shape
and arrangement of the annular permanent magnets of the permanent magnet
assemblies 160 and 162 of FIGS. 13-15 are not limited to the shapes and
arrangements disclosed hereinabove and that the number shapes and
arrangement of the annular permanent magnets included in the permanent
magnet assemblies 160 and 162 of FIGS. 13-15 may be varied and modified in
accordance with factors such as, inter alia, the required magnitude and
degree of homogeneity of the main magnetic field within the imaging volume
177, the desired dimensions of the permanent magnetic assemblies 160 and
162, the required size of the imaging volume 177, the degree of
homogeneity of the magnetic field provided by the electromagnetic flux
generator, the magnetization value of the permanent magnet material
available for constructing the annular permanent magnets and a variety of
other manufacturing considerations such as the weight and cost of
construction of the permanent magnetic assemblies.
Furthermore, while the preferred embodiments of the permanent magnet
assemblies disclosed hereinabove illustrate annular or disc-like permanent
magnets shaped as structures which are circularly symmetric with respect
to the z-axis 188 of the magnetic apparatus 150, other preferred
embodiments of the present invention may use other suitable shapes of
permanent magnet assemblies, provided that the shapes of the permanent
magnet assemblies are radially symmetric with respect to the z-axis 188.
For example, the disc-like permanent magnet 200 of FIG. 16 which has a
circular cross-section in a plane (not shown) perpendicular to the z-axis
188 may be replaced by a right regular polygonal prism having N sides,
wherein the number of sides N is large enough to provide the magnetic
field homogeneity requested within the imaging volume 177 for imaging.
Preferably, the number of sides is N.gtoreq.8, but other numbers may be
used depending, inter alia, on the requested degree of homogeneity of the
magnetic field within the imaging volume 177, and on the precise
geometrical shape of the electromagnetic coils within the electromagnet
assemblies 150 and 152. In another non-limiting example, the annular
permanent magnet 204 of FIG. 16 may be replaced by a right regular
polygonal ring structure having N sides, wherein the number of sides N is
large enough to provide the magnetic field homogeneity requested within
the imaging volume 177 for imaging. Preferably, the number of sides is
N.gtoreq.8, but other numbers may be used depending, inter alia, on the
requested degree of homogeneity of the magnetic field within the imaging
volume 177, and on the precise geometrical shape of the electromagnet
coils within the electromagnet assemblies 150 and 152.
Furthermore, while the electromagnet coils within the electromagnet
assemblies are preferably coils having a circular cross-section, in
accordance with another preferred embodiment of the present invention any
suitable coil shape may be used as long as the finally achieved degree of
homogeneity of magnetic field within the imaging volume 177 is sufficient
for performing MRI. For example, the electromagnet coils (not shown) may
have a regular polygonal ring like structure having N sides, wherein the
number of sides N is large enough to provide the magnetic field
homogeneity requested within the imaging volume 177 for imaging.
Preferably, the number of sides is N.gtoreq.8, but other numbers of sides
may be used depending, inter alia, on the requested degree of homogeneity
of the magnetic field within the imaging volume 177, and on the precise
geometrical shape of the annular and disc-like permanent magnets included
within the permanent magnet assemblies 160 and 162.
It will be appreciated that various modifications to the above-described
embodiment will be apparent to those of ordinary skill in the art in light
thereof. The above embodiments are provided by way of illustration and not
by way of limitation.
Top